Semiconductor image sensors are used to detect radiation such as visible light. A complementary metal-oxide-semiconductor (CMOS) image sensor (CIS) and charge-coupled device (CCD) are applied in various applications, such as cameras, mobile phones, and laptops for image capturing. The CIS utilizes an array of pixels in a substrate, including transistors and photodiodes to absorb photons received at the substrate and to convert the photons into an electrical charge. Analog signals resulting from the electrical charge are amplified and an analog-to-digital converter converts the amplified signals into digital signals. Afterward, a plurality of processes of color interpolation such as color correction, gamma correction and white balance are performed for image refining or compressing. In comparison with a front-side illumination CIS (FSICIS), a back-side illumination CIS (BSICIS) allows incident light to penetrate from a back side of a substrate, i.e., a side opposite to interconnect structures. In such a way, image sensor operation is improved as the BSICIS captures more photons of an image signal than the FSICIS due to reduced reflection of incident radiation by the interconnect structures.